This invention relates to an optical system and, more particularly, an illumination optical system for directing light from a light source to a surface to be illuminated. The present invention is suitably applicable, for example, to an illumination optical system in an exposure apparatus used in a photolithographic process for the manufacture of fine patterns such as semiconductor devices, for example.
In exposure apparatuses used in a photolithographic process for the manufacture of fine patterns such as semiconductor devices like semiconductor chips (e.g., ICs), liquid crystal panels, CCDs, thin magnetic heads, and micro-machines, for example, a resolving power much higher than that attained conventionally is required to meet further miniaturization of the pattern. If k1 is a constant which meets the process, the wavelength of exposure light is xcex, the numerical aperture of a projection optical system of an exposure apparatus is NA, and the resolving power is R, we obtain:
R=k1xc2x7xcex/NA.
It is seen from this equation that, to obtain a higher resolving power, the NA of the projection optical system should be enlarged and the wavelength of the light source should be shortened. In practice, the wavelength of exposure light used in exposure apparatuses has been changed from i-line (365 nm) to a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm). A further shortening of the wavelength is expected, such as the use of an F2 laser (157 nm), for example.
On the other hand, inside the exposure apparatus, in an illumination optical system for directing light from a light source to a reticle surface (surface to be illuminated) having a pattern formed thereon, due to the structure of the apparatus, it is necessary that light is deflected plural times. Conventionally, deflecting means therefor uses a mirror which comprises a substrate having a dielectric multilayered film formed thereon to attain a high reflection factor. Such a dielectric multilayered film mirror comprises an alternate accumulation of a film (H) having a higher refractive index (nH) and an optical film thickness corresponding to one-fourth (xc2xc) of the center wavelength of the exposure light, and a film (L) having a lower refractive index (nL) and an optical film thickness corresponding to one-fourth (xc2xc) of the center wavelength of exposure light. With the alternate accumulation of these layers, the reflection factor to incident light is increased.
As regards such a dielectric multilayered film mirror, it should have a sufficiently high reflection factor to the light of wavelength used as well as a sufficiently high durability thereto. However, as the wavelength used becomes shorter, from i-line (365 nm) to a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm), it becomes difficult to attain a sufficiently high reflection factor with such a dielectric multilayered film mirror.
This causes a decrease of illuminance of exposure light in an exposure apparatus which uses a light source of a short wavelength, and an increase in exposure time. It results in a lower throughput of the exposure apparatus or a failure in attaining a sufficient light quantity necessary for performing alignment measurement, for example.
On the other hand, the light to be supplied by a KrF excimer laser or an ArF excimer laser is an approximately linearly polarized light. The linearly polarized light has a high coherency, and the imaging performance differs with the direction of polarization. For these reasons, when linearly polarized light is used directly as exposure light, there is a possibility that non-uniform exposure results from an interference fringe or that the resolution differs with the direction of polarization.
It is accordingly an object of the present invention to provide an optical system by which light can be deflected while a high reflection factor is retained even with respect to light of a short wavelength, and by which depolarization of linearly polarized light can be accomplished.
In accordance with an aspect of the present invention, there is provided an optical system, comprising: an internal reflection type optical member effective to change a direction of advancement of an approximately linearly polarized light on the basis of total reflection; and a depolarizing member for dissolving linear polarization of light emitted from said optical member.
In one preferred form of this aspect of the present invention, said optical member may have a light entrance surface arranged so that a P-polarized component is incident thereon with a Brewster angle.
The optical member may have a light exit surface arranged so that a P-polarized component is emitted therefrom with a Brewster angle.
At the whole reflection surface of said optical member, the approximately linearly polarized light may be incident with an angle not smaller than a critical angle.
When an angle defined between a light entrance surface and a light exit surface of said optical member is xcex8 and an incidence angle of light incident on the while reflection surface of said optical member is A2, said optical member may satisfy a relation xcex8+2xc2x7A2=180 (deg).
The angle xcex8 defined between the light entrance surface and the light exit surface of said optical member may be 90 deg.
The approximately linearly polarized light to be deflected by said optical member may have an opening angle at one side, not greater than 3 deg.
The optical member may have a light entrance surface and a light exit surface each having an anti-reflection film formed thereon.
The optical member may have a light entrance surface and a light exit surface each being over-coated with a material having a refractive index lower than that of said optical member.
The optical system may further comprise a mirror having a dielectric film, wherein, for deflection of the approximately linearly polarized light, when the light is P-polarized light with respect to a light entrance surface of said mirror having the dielectric film or said optical member, said optical member may be used for the deflection, and wherein, when the light is S-polarized light, said mirror may be used for the deflection.
In accordance with another aspect of the present inventions there is provided an illumination optical system, comprising: an internal reflection type optical member effective to change a direction of advancement of an approximately linearly polarized light on the basis of total reflection; and a depolarizing member for dissolving linear polarization of light emitted from said optical member.
In one preferred form of this aspect of the present invention, the illumination optical system may further comprise a fly""s eye lens for receiving light having its linear polarization dissolved by said depolarizing member and for producing a plurality of secondary light sources at a light exit surface thereof, and a lens system for superposing light beams from the secondary light sources one upon another on the surface to be illuminated.
The optical member may have a light entrance surface arranged so that a P-polarized component is incident thereon with a Brewster angle.
The optical member may have a light exit surface arranged so that a P-polarized component is emitted therefrom with a Brewster angle.
At the whole reflection surface of said optical member, the approximately linearly polarized light may be incident with an angle not smaller than a critical angle.
When an angle defined between a light entrance surface and a light exit surface of said optical member is xcex8 and an incidence angle of light incident on the while reflection surface of said optical member is A2, said optical member may satisfy a relation xcex8+2xc2x7A2=180 (deg).
The angle xcex8 defined between the light entrance surface and the light exit surface of said optical member may be 90 deg.
The approximately linearly polarized light to be deflected by said optical member may have an opening angle at one side, not greater than 3 deg.
The optical member may have a light entrance surface and a light exit surface each having an anti-reflection film formed thereon.
The optical member may have a light entrance surface and a light exit surface each being over-coated with a material having a refractive index lower than that of said optical member.
The optical system may further comprise a mirror having a dielectric film, wherein, for deflection of the approximately linearly polarized light, when the light is P-polarized light with respect to a light entrance surface of said mirror having the dielectric film or said optical member, said optical member may be used for the deflection, and wherein, when the light is S-polarized light, said mirror may be used for the deflection.
In accordance with a further aspect of the present invention, there is provided an exposure apparatus, comprising: an illumination optical system including (i) an internal reflection type optical member effective to change a direction of advancement of an approximately linearly polarized light on the basis of total reflection, and (ii) a depolarizing member for dissolving linear polarization of light emitted from said optical member; and a projection optical system for projecting a pattern, formed on a surface to be illuminated by said illumination optical system, onto a substrate.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: illuminating a surface to be illuminated, by use of an illumination optical system which includes (i) an internal reflection type optical member effective to change a direction of advancement of an approximately linearly polarized light on the basis of total reflection, and (ii) a depolarizing member for dissolving linear polarization of light emitted from the optical member; and transferring a pattern, formed on the surface to be illuminated, onto a substrate by use of a projection optical system.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.